In optical material fields, recently, plastic materials are used a lot for optical materials, such as a spectacle lens, a prism, an optical fiber, an information recording substrate and a filter, and for coating materials and adhesives for the optical materials, since they are lightweight and are abundant in toughness. Particularly, with respect to materials for a spectacle lens a coating layer, an optical filter and the like, a plastic material having a high refractive index is required.
As a material having a high refractive index, there are proposed a copolymer of a polythiol compound and a polyisocyanate (JP-B-4-58489, JP-B-5-148340), a ring-opened polymer of a polyepisulphide compound (JP-A-9-110979), a polyenic compound (JP-A-9-110983), a poly(thio)isocyanate compound (JP-A-9-110955, JP-A-6-5323) and a polyepisulfide compound (JP-A-10-298287). However, heat-curing is a method for molding these materials and problems are that a molding time is long and that molding at low temperatures is difficult. Therefore, there are increasingly required photocurable materials which are moldable by molding in a short time and molding at a low-temperature. Since a polythiol compound does not have self-polymerizability, the compounding amount thereof is limited. Further, when the compounding amount of the polythiol compound increases, a problem is that an obtained cured product becomes flexible due to a sulfide bond (—S—) formed at the time of polymerization.
As an example of a photocurable resin having a high refractive index, a resin (to be referred to as “enic-thiol resin” hereinafter) formed of a combination of an enic compound and a thiol compound is proposed. Since the thiol compound has a sulfur atom having a high atom refractive index in a molecule, a cured product obtained therefrom has a high refractive index. Further, the cured product has high impact resistance thanks to addition linkage of a thiol group and an unsaturated group. However, when the content of the thiol compound is increased for increasing the refractive index sufficiently, an obtained cured product becomes a rubbery state so that sufficient hardness can not be obtained. For obtaining a cured product having a high refractive index and high hardness, there has been demanded a polyfunctional, preferably tetrafunctional or more functional, polythiol which has a sufficiently high refractive index and is easily apt to form a crosslinking structure.
Further, concerning the enic compound being a partner of the thiol compound, it is required to select an adequate compound in consideration of a balance of photocurable properties, hardness, or the like, in addition to the refractive index.
Polyenic compounds having two or more reactive double bonds such as an acryloyl group, a methacryloyl group, a vinyl group or an allyl group are enumerated as a photocurable material. However, for example, generally-known epoxy acrylate (JP-A-8-176243) or urethane acrylate (JP-A-5-134101) has a defect of a low refractive index. Therefore, polyenic compounds having higher refractive index have been required.